The differential charge pump is a basic building block of many types of analog circuits, including frequency control circuits such as phase locked loops (PLLs). The PLL accurately controls the frequency and phase of an output oscillatory signal so as to match that of an input oscillatory signal. In this way, the output signal precisely tracks, and essentially duplicates, the input signal. The oscillatory signals may be either digital or analog. FIG. 4 shows a block diagram of a conventional PLL. The main process in the loop is a charge pump and integrator/filter combination 408 and a voltage controlled oscillator (VCO) 412. A phase-frequency detector (PFD) 404 provides a differential voltage that is proportional to the phase error. The charge pump and filter 408 condition this error signal to stabilize the PLL loop before sending it to the VCO. The VCO 412 generates an output oscillatory signal whose frequency and phase are proportional to an input differential voltage. An out_phase signal from the VCO 412 is fed back to the PFD 404 and subtracted from an in_phase signal to yield the phase error. Assuming that the control loop can be properly initialized, out_phase should precisely track in_phase in the steady state.
FIG. 5 illustrates the PFD 404 that detects the phase error between two digital signals in_phase and out_phase. The flip-flops and NAND gate in the PFD 404 are configured so that up_sig is asserted high if a rising edge of in_phase leads the corresponding rising edge of the out_phase. On the other hand, if the rising edge of in_phase lags the corresponding rising edge of out_phase, then dn_sig is asserted high. Asserting up_sig indicates that the phase of the oscillatory output signal (see FIG. 4 momentarily) should be increased, whereas dn_sig indicates that the phase should be decreased. Note that both up_sig and dn_sig are deasserted when the lagging rising edge has been detected. These control signals represent the phase error which is the desired change to be implemented in the oscillatory output signal.
The control signals up_sig and dn_sig are translated into a relatively slow changing differential voltage by the charge pump and filter combination 408. This differential voltage is then used to control the frequency and phase of the oscillatory output signal through the VCO 412. FIG. 6 depicts the charge pump and filter combination 408 in block diagram form. The charge pump 408 includes four current generators 422 . . . 428. Each one alternatively sources or sinks current from one of a pair of filter and bypass nodes. The current generators are connected to their respective filter/bypass nodes by solid state switches. The switches for current generators 424 and 428 are controlled by up_sig, whereas the switches for current generators 422 and 426 are controlled by dn_sig (see FIG. 5 momentarily). In typical operation, the voltages on the filter and filter nodes are subjected to rapid differential corrections: increases in response to up_sig and decreases in response to dn_sig. A loop filter 432 integrates/filters these rapid changes to yield a slow changing differential voltage which is then supplied to the VCO 412. The bypass and bypass nodes to help maintain the transistors in the current generators continuously in their saturation region of operation while the generators are not connected to their respective filter nodes, thus providing a differential charge pump circuit whose output voltage in actual operation is more consistent with design values despite manufacturing variations.
The charge pump 408 also includes a common mode control circuit 450 for adjusting the common mode voltage of the filter and filter nodes to a desired level that is suitable for the VCO 412. The common mode control circuit 450 includes a set of current generators 462 and 464 that make corrections directly to the filter and filter nodes in response to a difference between desired and actual common mode levels. This approach to controlling the common mode level, however, has the serious drawback of introducing a low impedance path to a power supply node, i.e., ground, through the current generators 462 and 464, which in turn introduces noise to the filter and filter nodes and may cause the differential output voltage of the charge pump to drift through charge loss.